Category Archives: Fortnightly Weather Review

1.1 During May 2026, a mix of near- to above-average rainfall was recorded over most of Mainland Southeast Asia with a mix of below- to above-average rainfall for the Maritime Continent (Figure 1). Parts of southwestern and central Mainland Southeast Asia recorded above-average rainfall based on both datasets (GSMaP-NRT and CMORPH-CRT). The western and southeastern Maritime Continent recorded mostly above-average rainfall, with most of the northeastern part recording below-average rainfall and the central Maritime Continent a mix of below- to above-average rainfall. The largest positive (wetter) anomalies were recorded over the southern Malay Peninsula and Papua for both datasets, with the largest negative (drier) anomalies over the central and northern parts of the Philippines.

1.2 The observed rainfall anomaly pattern of above-average rainfall over southwestern Mainland Southeast Asia to a lesser extent over the southern Malay Peninsula is consistent with the predictions from the subseasonal weather outlooks for May 2026 (27 April – 10 May 2026, 11 – 24 May 2026 and 25 May – 7 June 2026). The wet anomalies over the western Maritime Continent and the dry anomalies over the Philippines are consistent with the seasonal outlook for May 2026, however, the seasonal outlook predicted below-normal rainfall as the dominant tercile for southwestern Mainland Southeast Asia.

Figure 1: Rainfall anomalies for May 2026 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded eastern Mainland Southeast Asia, with below- to near-average temperature for northwestern Mainland Southeast Asia (Figure 2). Near-to above-average temperature was recorded for much of the Maritime Continent. The warmest anomalies (1°C – 2°C above average) were recorded over parts of northern Cambodia, southern Lao PDR and northern Philippines. The coolest anomalies (-1°C – -2°C below average) were recorded over parts of central and northern Myanmar. Cooler anomalies (-0.5°C – -1°C below average) were also recoded over central Thailand and Papua.

Figure 2: Temperature anomalies for May 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO) was active over the Indian Ocean and Western Pacific during much of May. During the first two weeks of May, an MJO signal was present over the Indian Ocean (Phases 2 and 3), before decaying and becoming inactive during the third week of the month. An MJO signal then emerged over the Western Pacific (Phases 6 and 7) in the last week of May. At this time of the year, Phases 2 and 3 tend to bring wetter conditions to the western and central Maritime Continent, with Phases 6 and 7 tending to bring drier conditions to the eastern and central Maritime Continent. This is somewhat in line with wetter conditions over parts of the western Maritime Continent and drier conditions over the northwestern Maritime Continent in Figure 1.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 The El Niño – Southern Oscillation (ENSO) was neutral in April, with the Indian Ocean Dipole (IOD) also in the neutral state.

1.1 During February 2026, above-average rainfall was recorded over much of the northern half of the Maritime Continent with near-average rainfall over most of Mainland Southeast Asia (Figure 1). While above-average (wetter) anomalies are observed distributed over much of northern half of the area for both datasets (GSMaP-NRT and CMORPH-BLD), there is disagreement elsewhere over the Maritime Continent. GSMaP-NRT recorded above-average anomalies but CMORPH-BLD recorded mainly below-average (drier) anomalies for much of the southern half of the Maritime Continent. There were no notable anomalies over most of Mainland Southeast Asia, in line with the current dry season, with some regions of above-average rainfall over parts of Viet Nam. The largest positive (wetter) anomalies were recorded over southern Philippines for both datasets, with the largest negative (drier) anomalies over southern Borneo for CMORPH-BLD.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the northeastern Maritime Continent, where the highest anomalies are recorded, is consistent with the predictions from the subseasonal weather outlooks for February 2025 (2 – 15 February 2026, and 16 Febraury – 1 March 2025). The subseasonal weather outlooks predicted a drier central Maritime Continent, which is more consistent with the recorded rainfall from CMORPH-BLD. The observations match the most likely tercile from the seasonal outlook for February 2026 for northeastern Maritime Continent where the rainfall observed the largest positive anomalies.

Figure 1: Rainfall anomalies for February 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over most of Mainland Southeast Asia, with below- to near-average temperature for southeastern Mainland Southeast Asia (Figure 2). For the Maritime Continent, near-average temperature was generally recorded. The warmest anomalies (2°C – 3°C above average) were recorded over northern Viet Nam, while the coolest anomalies (0.5°C – 1°C below average) were recorded over Papua and central Cambodia.

Figure 2: Temperature anomalies for February 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of January, the Madden-Julian Oscillation (MJO) was inactive based on the RMM diagram (Figure 3). The signal emerged over the Western Pacific (Phase 6) in Week 2, strengthening with little eastward propagation in Phase 6 until middle of Week 3 when it weakened slightly and started again propagating eastwards. By Week 4, the MJO signal was in Phase 7 of the Western Pacific where it stalled and weakened at the end of the month. At this time of the year, Phases 6 and 7 tend to bring drier conditions to the western and southern Maritime Continent, in line with the observed rainfall in January also showing drier conditions for much of the Maritime Continent.

 

Figure 3: The Madden-Julian Oscillation (MJO) was inactive for much of February based on the RMM diagram (Figure 3). The signal decayed over the Western Hemisphere (Phase 8) at the beginning of the month , before emerging shortly in the middle of February over the Indian Ocean (Phases 2 and 3) and decaying again after a week. The MJO signal remained indiscernible for much of Week 4, before emerging over the Maritime Continent (Phases 4 and 5) at the end of the month. At this time of the year, Phases 2 and 3 as well as Phases 4 and 5 tend to bring wetter conditions to the Maritime Continent, with the observed rainfall in February also showing wetter conditions (data from the Bureau of Meteorology, Australia).

 

2.2 La Niña conditions persisted and continued weakening in February 2026. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) is currently in the neutral state.

1.1 During January 2026, below- to near-average rainfall was recorded over much of the Maritime Continent with near-average rainfall over Mainland Southeast Asia (Figure 1). For the Maritime Continent, most of the equatorial region recorded below-average (drier) anomalies, with scattered spots of near- to above-average (wetter) anomalies for the southern and northeastern regions for both datasets (GSMaP-NRT and CMORPH-CRT). There were no notable anomalies over Mainland Southeast Asia, in line with the current dry season. The largest positive (wetter) anomalies were recorded over central Philippines, with the largest negative (drier) anomalies over western Borneo.

1.2 The observed rainfall anomaly pattern of below-average rainfall over much of the equatorial region and the above-average rainfall over northeast Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for January 2025 (22 December 2025 – 4 January 2026, 5 – 18 January 2025, and 19 January – 1 February 2026). The observations match the most likely tercile from the seasonal outlook for January 2026 for central and northeastern Maritime Continent where the rainfall observed the largest drier and wetter anomalies, but differ for much of the rest of the Maritime Continent.

Figure 1: Rainfall anomalies for January 2026 based on GSMaP-NRT data (left) and CPC CMORPH-CRT data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over the eastern Maritime Continent and parts of northern Mainland Southeast Asia, while below-average temperature was recorded over central and southern Mainland Southeast Asia, with near-average temperature for much of the rest of Southeast Asia (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over western Myanmar, while the coolest anomalies (1°C – 2°C below average) were recorded over Cambodia, southern Viet Nam and central Thailand.

Figure 2: Temperature anomalies for January 2026 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of January, the Madden-Julian Oscillation (MJO) was inactive based on the RMM diagram (Figure 3). The signal emerged over the Western Pacific (Phase 6) in Week 2, strengthening with little eastward propagation in Phase 6 until middle of Week 3 when it weakened slightly and started again propagating eastwards. By Week 4, the MJO signal was in Phase 7 of the Western Pacific where it stalled and weakened at the end of the month. At this time of the year, Phases 6 and 7 tend to bring drier conditions to the western and southern Maritime Continent, in line with the observed rainfall in January also showing drier conditions for much of the Maritime Continent.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 La Niña conditions persisted in January 2026, although key atmospheric indicators show signs of the La Niña conditions weakening. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) is currently in the neutral state.

1.1 During December 2025, a mix of below- to above-average rainfall was recorded over the Maritime Continent with near-average rainfall over Mainland Southeast Asia (Figure 1). For the Maritime Continent, most of the equatorial region recorded above-average (wetter) anomalies with the surrounding region recording predominately below-average (drier) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended). There were no notable anomalies over Mainland Southeast Asia, in line with the current dry season. The largest positive (wetter) anomalies were recorded over parts of Peninsular Malaysia and Borneo, with the largest negative (drier) anomalies over the northern Malay Peninsula and the southern Philippines.

1.2 The observed rainfall anomaly pattern of below-average rainfall over parts of the Maritime Continent outside of the equatorial region is consistent with the predictions from the subseasonal weather outlooks for December 2025 (24 November – 7 December 2025, 8 – 21 December 2025, and 22 December – 4 January 2026) although the above-average anomalies for most of the equatorial region were not predicted. The observations differ from the most likely tercile from the seasonal outlook for December 2025, which predicted an increase in chance of above-normal rainfall over parts of the northeastern and southern Maritime Continent.

Figure 1: Rainfall anomalies for December 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Above-average temperature was recorded over northeastern Mainland Southeast Asia and the northeastern Maritime Continent, with near-average temperature for much of the rest of Southeast Asia (Figure 2). The warmest anomalies (1°C – 3°C above average) were recorded over northern and central Viet Nam, while the coolest anomalies (0.5°C – 1°C below average) were recorded over northern Myanmar.

Figure 2: Temperature anomalies for December 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of December, the Madden-Julian Oscillation (MJO) was present over the Western Pacific (Phase 7) based on the RMM diagram (Figure 3). The signal propagated eastwards, weakening over Western Hemisphere (Phase 8) and entering the unit circle by Week 2. For the rest of December there was no sustained MJO signal based on the RMM index. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 La Niña conditions persisted in December 2025. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year. The Indian Ocean Dipole (IOD) returned to neutral in December 2025.

1.1 During November 2025, a mix of below- to above-average rainfall was recorded for Southeast Asia (Figure 1). For the Maritime Continent, most of the northeastern and northwestern regions recorded positive (wetter) anomalies in both datasets (GSMaP-NRT and CMORPH-Blended), with a mix of below- to above-average rainfall recorded over the equatorial region. Over Mainland Southeast Asia, above-average rainfall was recorded mainly over the central and eastern regions. Generally, CMORH-Blended (Figure 1, right) recorded more extensive negative (drier) anomalies over the equatorial region. The largest positive (wetter) anomalies were recorded over the northwestern Maritime Continent , with the largest negative (drier) anomalies over central Borneo.

1.2 The observed rainfall anomaly pattern of above-average rainfall over the Mainland Southeast Asia and below-average rainfall over the central Maritime Continent is consistent with the predictions from the subseasonal weather outlooks for November 2025 (27 October – 9 November 2025, 10– 23 November 2025, and 24 November – 7 December 2025).The observations are also somewhat consistent with the seasonal outlook for November 2025, which predicted an increase in chance of above-normal rainfall over most of Mainland Southeast Asia and the northeastern Maritime continent, with an increase in chance of below-normal rainfall over the central Maritime Continent.

Figure 1: Rainfall anomalies for November 2025 based on GSMaP-NRT data (left) and CPC Global Unified Gauge-based Analysis data (right). The climatological reference period is 2001-2024. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Below-average temperatures were recorded over most of Mainland Southeast Asia in November 2025, while near- to above-average temperatures were recorded over most of Maritime Continent (Figure 2). The warmest anomalies (1°C – 2°C above average) were recorded over east central Sumatra, while the coolest anomalies (0.5°C – 1°C below average) were recorded over central Mainland Southeast Asia.

Figure 2: Temperature anomalies for November 2025 based on ERA-5 reanalysis. The climatological reference period is 2001-2024. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 At the start of November, the Madden-Julian Oscillation (MJO) was present over the Maritime Continent (Phase 5) based on the RMM diagram (Figure 3). It stalled over the Western Pacific (Phase 6) during Week 2 and then weakening in Phase 6 during the third week. The MJO strengthened again in Phase 6 towards the end of Week 3 and resumed its eastward propagation, reaching phase 7 in Week 4. At this time of the year, Phases 1 and 2 tend to bring drier conditions to the eastern Maritime Continent, while Phases 4 and 5 tend to bring wetter conditions to much of the Maritime Continent. However, these patterns are not clear in Figure 1, indicating other drivers and weather systems likely had a stronger influence on the region’s rainfall.

 

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

 

2.2 A negative Indian Ocean Dipole (IOD) was present in November 2025. Negative IOD events tend to bring wetter conditions to parts of the region, particularly the southern Maritime Continent, in line with the positive rainfall anomalies in Figure 1. La Niña conditions were also present. La Niña events tend to bring wetter-than-average conditions to much of the Maritime Continent during this time of the year.